Electron microscopes are used for examining an object by directing a primary electron beam towards the object to be examined, whereupon electrons emanating from the object are guided to a detector as a secondary electron beam, and detected there.
For electron microscopes, it is generally desirable to deflect the primary and/or the secondary electron beam in a precise manner, e.g. about an angle of 10° or more; or to separate an electron beam from a counter-propagating electron beam, for being able to separately manipulate these beams. For an electron microscope operating in 180°-deflection, or an electron microscope with a mirror corrector, this typically means that both beams have to pass a deflecting field of a beam separator.
For beam guiding, magnetic fields are usually employed, which often have to fulfill high demands concerning their homogeneity, stability and edge effects. Metal plates with a high magnetic conductivity, usually of an iron-nickel-alloy, can be placed at a fixed distance opposite from one another as pole plates for generating these magnetic fields. Coils are fitted into the surfaces of the pole plates, which coils may be energized by a flow of current. A magnetic field which is homogeneous to a high degree of approximation is then generated on the areas surrounded by the coil wires. Such a device is known from German Patent No. 102 35 981.
The demands on the stability and accuracy of the geometry of the pole plates can be quite high, in particular for the electron microscopes with a mirror corrector mentioned above.